While the aspiration of fluids from sealed containers may be required in various industrial and laboratory settings, such procedures are most frequently performed in medical laboratories and other medical settings where blood, other bodily fluids or some reactant or dilutant is aspirated from a sealed test tube, bottle, vial or other container and is then dispensed into a cuvette or other vessel to have appropriate test procedures performed thereon. The containers in which the fluid is initially stored are frequently at a significant initial negative pressure as to both facilitate the drawing of blood into the container and to also inhibit the escape of blood from the container. However, even where the container is not negatively pressurized, the pressure in the container may vary significantly as a result of factors such as changes in the volume of fluid in the container, changes in the temperature of the fluid and on ambient temperature, and leakage through the seals. Where the blood in the container is taken directly from an individual, the pressure may also vary depending on the blood pressure of the individual from whom the blood was obtained.
Therefore, the pressure in the container at any given time can vary significantly, either positively or negatively, but generally negatively, from the ambient or atmospheric pressure outside the container. This pressure variation can cause errors in the quantity of fluid dispensed when standard fluid aspirating and dispensing procedures are utilized. In particular, the standard procedure for aspiration is to insert the probe into the container, aspirate a small gas bubble into the probe to separate the fluid being aspirated from the water or other fluid being acted on by the pump in the system, aspirate at least one small separator sample of fluid, aspirate another separator air bubble(s) and then aspirate the sample. The probe is them removed from the container and the pump of the system is operated to dispense a precise quantity of the sample.
However, if the pressure in the container is less than that outside the container, the air bubbles in the probe will shrink slightly when the probe is removed from the container, resulting in a small air gap at the tip of the probe. Therefore, when the pump attempts to dispense a precise quantity of fluid, the initial dispensing will be of air, resulting in a slight underdispensing of the blood or other fluid. Depending on the pressure differential, the size of the bubbles and the size of the sample, this underprovision of sample may be 10% or more of the desired sample and could lead to inaccurate results in medical tests being performed. Similarly, if the pressure in the container is greater than the external ambient pressure, the bubbles will expand when the probe is removed from the container, causing a drop of fluid to form at the end of the probe which can result in an over dispensing of liquid with similar undesirable results.
In the past, to the extent these problems have dealt with, they have been dealt with by assuring that the pressure in the container is equalized with the external pressure when the probe is inserted in the container, while the sample is aspirated and when the probe is removed. One way of accomplishing this objective is for example taught in U.S. Pat. No. 4,951,512 which utilizes a cannula surrounding the probe which is inserted through the seal with the probe or prior to insertion of the probe and remains in place through the aspirating operation. Air flowing through the space between the probe and cannula keeps the pressure equalized between the inside and outside of the container. While this mechanism solves the problems indicated above, it also has a number of limitations. First, when the open cannula is initially inserted into the container, blood can shoot out through cannula. With today's AIDS consciousness, any procedure which results in blood splattering or aerosol is undesirable. Second, the device is relatively complex to make and use and is much more complicated to clean than if only a probe were utilized. Finally, the cannula makes a much larger hole in the seal than would be made by the probe alone, therefore requiring a greater force to penetrate and increasing the chances an incomplete resealing of the container after the probe and cannula are removed.
It would therefore be desirable if the problem of inaccurate fluid dispensing resulting from pressure differentials between the inside and outside of the sealed container could be eliminated without the use of an additional cannula around the probe to provide continuous pressure equalization.